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Gel time of different volume fractions of polystyrene particles
Scattering of light in colloid
08/03/06
Rakshya Khatiwada
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Outline
• Introduction
• Apparatus
• Calibration
• Sample Preparation
• Measurements and Data Analysis
• Results
• Conclusion
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Introduction
• 3D solid objects don’t coalesce like liquids
• They form fractals with some different dimension
• In dilute systems, including aerosols and colloids the dimension is 1.8
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• What is gelation of colloids?
1. Network of smaller particles/monomers2. The particles exhibiting Brownian motion
stop/slow down
• How do we know when the gel point is reached?
The intensity of light doesn’t change much because the network stops growing
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What is polystyrene?
• Micro spheres of 24nm diameter
• About same charge
• MgCl2 forms ions which screen the coulomb charge repulsion allowing them to stick together with van der Waal’s force
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Calibration Apparatus• Used single slit (D=10µm)
Sample
CCD camera
Filter
Small mirror
488nm laser beam
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Single slit diffraction: 2))(0()( Sin
II 2
Dq
2)
4(
Sinq
Scattering wave vector Unit: 1/m gives the size of the aggregate
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Sample preparation
• Different volume fractions of polystyrene:
1. 9.14E-42. 7.03E-43. 5.41E-44. 4.16E-45. 3.20E-46. 2.46E-4
• 35mM of MgCl2
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50μl of Polystyrene+50μl of MgCl2
Sample Holder
MgCl2 Polystyrene
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Measurements and Data Analysis
Intensity vs. wave vector q
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Gel time vs. volume fractionVolume fraction
Theoretical gel time (minutes)
Experimental gel time (minutes)
9.14E-4 0.300
7.03E-4 0.629
5.41E-4 1.21
4.16E-4 2.33
3.20E-4 4.50
2.46E-4 8.69
326 333 539 554
566 1078
Theoretical gel time:
)3/(3
DDvg f
K
at
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Results Gel time vs. volume fraction
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Conclusion
• Reproducible data
• Lower volume fractions, longer gel time
• Experimental gel times longer than theoretical
• Literature values compared to our theory are consistent with our conclusions here.
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Possibilities
• Maybe theory has simplified so many details (example: same cluster size)
• Maybe stoppage of system to evolve is not good indication of gel time.
• Need a better way of finding gel point
• Like Dynamic light scattering (can see particles moving)
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Acknowledgement
Supervisor: Tahereh Mokhtari & Dr. Christopher Sorensen
Thanks to Rajan Dhoubhadel & Hao Yan